Introduction

The kidney processes 20%–25% of the blood volume of the entire cardiac output and is one of the organs most affected by exercise-induced blood redistribution [13, 17]. Renal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80–180 mmHg). However, attenuated renal autoregulation contributes to renal injury in many models of renal, diabetic, and hypertensive diseases [21]. Furthermore, renal autoregulatory functions have been reported to be reduced in type 2 diabetes mellitus [4]. Diabetes and hypertension are major causes of chronic kidney disease and are both associated with impaired autoregulation of RBF and increased transmission of arterial pressure variations to the glomerular capillaries [20]. Particularly, attenuated renal autoregulation and associated blood flow can cause renal injury in patients with underlying diseases. Guidelines for renal rehabilitation propose that patients should perform aerobic exercises (moderate-intensity exercises) [33]. While moderate-intensity exercises are safe, high-intensity exercises can increase the biomarkers of muscle and kidney damages, including serum creatine kinase (CK, urine myoglobin, and creatinine levels [15, 28]). Several studies have reported a decrease in RBF in healthy subjects after high-intensity exercises, like bicycle exercise and running [23]. However, whether diseased or healthy subjects can maintain RBF with moderate-intensity exercise [ventilation threshold (VT)] is unclear. It is crucial to confirm this in healthy subjects before confirming it in patients with kidney disease. Therefore, this study aims to confirm the changes in RBF in healthy subjects after moderat level exercise, which will have implications for the study of renal arterial blood flow in patients with renal failure.

Moderate-intensity exercises are often used based on the VT. We determined the VT value by measuring carbon dioxide excretion, ventilatory volume exhalation, and physiological autonomic turning point examination [27, 30]. Autonomic changes result in heart rate (HR) fluctuation, vascular changes, and increased sympathetic nerve activity, which causes constriction of renal arteries [26].

Therefore, the two hypotheses of this study are (1) RBF is maintained until VT and (2) RBF decreases after VT.

Methods

Participants and Procedure

Healthy participants without any underlying disease were recruited between November 2018 and January 2020. All test participants volunteered to participate in the study, were informed about the details of the study, and provided written consent to participate before enrollment. The trial was approved by the institutional review board of the Asahi University Hospital (approval No. 2018-05-01), and the trial was registered on 2019/1/24 (UMIN 000035598) [32].

Eligibility criteria included healthy volunteers aged 18 years or older without hypertension, pulmonary and cardiac disorders, cerebrovascular disease, other pre-existing diseases, and obesity, while exclusion criteria included the lack of respiratory cooperation and large amounts of intestinal gases [29].

Protocol and Assessment

A cardiopulmonary exercise test (CPX) was conducted on the first visit before VT (pre-VT), after VT (post-VT), and after the respiratory compensation point (RCP). On the second visit, participants performed a constant work-rate exercise test at three exercise intensities—pre-VT, post-VT, and RCP—for 3 min each. The RBF was measured immediately after each load (Fig. 1). CPX was performed using a cycle ergometer (STB-3400 Nihon Kohden). Oxygen uptake (\(\dot{V}\)O2), carbon dioxide production (\(\dot{V}\)CO2), and minute ventilation (\(\dot{V}\)E) were measured using an expired gas analyzer (Ae-310 Minato). The HR of the participants was monitored throughout the test using the stress system STS 2100 (Nihon Kohden). Systolic blood pressure (SBP) was monitored every minute using a stress test monitor by Sun Tech (Tango M2).

Fig. 1
figure 1

Intermittent incremental exercise. Participants perform a constant work-rate exercise test at three exercise intensities for 3 min each. The RBF was measured immediately after each load

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VT [30] is a value determined by the amount of carbon dioxide expelled during exhalation and ventilation. It acts as a turning point for the autonomic nervous system [34], and sympathetic nerve responses are increased post-VT.

We determined the ventilatory threshold using the ventilatory equivalent method (VEQ method) [30]. The exercise intensity examined in this study leads to an increase in the ventilatory equivalent of oxygen (\(\dot{V}\)E/\(\dot{V}\)O2) without a concurrent increase in the ventilatory equivalent of carbon dioxide (\(\dot{V}\)E/\(\dot{V}\)CO2) [30]. The RCP can be calculated from the increase in the elevation of the slope when \(\dot{V}\)E is plotted against \(\dot{V}\)CO2.

Ultrasonography for RBF Measurement

Recent advances in ultrasonography allow simultaneous assessments of blood flow velocity and the cross-sectional area (CSA) of blood vessels [18]. Several studies have also evaluated the value of color-coded duplex ultrasonography [18, 22] and the reproducibility of blood flow measurements [3]. RBF was determined using the formula: RBF (mL/min) = time-averaged flow velocity (TAV) (cm/s) × CSA (cm2) × 60 (beats/min) [18]. Ultrasound visualization of the renal arteries was performed using the HITACHI ALOKA Noblus (Hitachi Aloka, Tokyo, Japan) flank view approach.

Statistical Analysis and Sample Size

Analyses of HR, SBP, TAV, CSA, and RBF were performed for all participants and analyzed immediately after CPX. A normality test (Shapiro–Wilk test) was performed to confirm normality. Data are presented as mean ± SE (standard error). Bonferroni’s test was used to compare the variables to the baseline data. Statistical significance was set at P < 0.05. All analyses were performed using IBM SPSS Statistics ver. 24 (IBM Corp., Armonk, NY, USA). The sample size was calculated to generate 80% power and 5% alpha risk using G power version 3.1.9 software (Dusseldolf University, Düsseldorf, Germany), considering the number of cases based on previous studies. A sample size of ten participants was estimated to be statistically adequate, since changes in blood flow had been measured in 10–11 cases by previous studies [8, 18].

Results

The clinical characteristics and CPX results of the ten participants are presented in Table 1. The mean age of the participants was 31 years. All participants displayed normal ECG records during CPX. The peak \(\dot{V}\)O2 was 30.4 ± 3.3 mL/kg/min. The VT point was 66.8 ± 11.1 W (Table 1).

Table 1 Results of the cardiopulmonary exercise test (CPX)
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Change in HR and SBP

The HR of the participants increased linearly at pre-VT, post-VT, and RCP, respectively. There was a significant increase in HR pre-VT, post-VT, and RCP compared to the baseline (pre-VT 97.8 ± 10.8 beats/min, post-VT115 ± 12.6 bearts/min, RCP 147.9 ± 20.1 beats/min, P < 0.01 vs. baseline). The SBP did not show any significant change (pre-VT145 ± 4.8 mmHg vs. baseline131 ± 5.6 mmHg, P = 0.06) at pre-VT, but showed a significant increase at post-VT and RCP (post-VT166 ± 7.2 mmHg, RCP 207 ± 7.5 mmHg, P < 0.05 and P < 0.01 vs. baseline, respectively) (Fig. 2).

Fig. 2
figure 2

Changes in systolic blood pressure. The systolic blood pressure at pre-ventilation threshold did not show any significant change. However, there was a significant increase in systolic blood pressure at the post-ventilation threshold and respiratory compensation point

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Ultrasonography Data

The TAV at the baseline was 29.0 ± 7.8 cm/s. The TAV at pre-VT, post-VT, and RCP did not show any significant difference compared to the values at the baseline. In addition, significant differences were found in RCP (pre-VT, 30.0 ± 7.7 cm/s, P = 0.44; post-VT, 24.0 ± 5.5 cm/s, P = 0.50; RCP, 22 ± 3.8 cm/s, P < 0.05 vs. baseline) (Fig. 3). Moreover, CSA at pre-VT, post-VT, and RCP did not show significant differences compared to the CSA at baseline (pre-VT, 0.27 ± 0.06 cm2, P = 0.67; post-VT, 0.19 ± 0.05 cm2, P = 0.19; RCP, 0.15 ± 0.03 cm2, P = 0.05 vs. baseline) (Fig. 4).

Fig. 3
figure 3

Changes in the time-averaged flow velocity. No exercise intensity showed significant difference as compared to the time-averaged flow velocity at baseline

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Fig. 4
figure 4

Changes in cross-sectional area. No exercise intensity shows significant differences as compared to the cross-sectional area at baseline

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The RBF measured at baseline was 461 ± 142 mL/min. While the RBF at pre-VT was not significantly different from that at baseline (482 ± 142 mL/min; P = 0.82 vs. Baseline), significant differences were observed in the RBFs at post-VT when compared with baseline (289 ± 111 mL/min; P < 0.01). Similar trends were also seen for RBFs at the RCP (212 ± 56 mL/min; P < 0.01 vs. baseline) (Fig. 5). The RBF at post-VT and RCP was approximately 37% and 54% lower, respectively, than that at the baseline.

Fig. 5
figure 5

Changes in renal artery blood flow. The renal artery blood flow at post-VT and RCP is approximately 37% and 54% lower, respectively, than that at the baseline

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Discussion

We verified our hypothesis that VT-based moderate-intensity exercise lowers RBF in healthy subjects. Our findings showed that during exercise, RBF decreased immediately after VT and confirmed that RBF fluctuates at pre-VT, post-VT, and RCP. We found that (1) there was no change in the RBF at pre-VT, and (2) the RBF was significantly lower at post-VT and RCP. Compared with the resting phase (baseline), RBF decreased by approximately 63% at post-VT and 46% at RCP. According to Suzuki et al. [31], RBF decreases by 53.4% after exercise, and their values at RCP coincide with the findings of earlier studies.

Exercise-induced proteinuria is strictly related to exercise intensity, not exercise duration [2]. While some reports indicate that high-intensity loading does not cause renal dysfunction in young diabetic patients [16], high-intensity loading is not recommended as it leads to organ damage, including increased renal damage; while exercise, including aerobic exercise, improves glomerular filtration and reduces risk factors of cardiovascular disease in patients with kidney disease [14]. In addition, oxygen transport in patients with renal disease is proportional to total RBF, and it directly depends on blood flow regulation. In some patients with renal diseases, high-intensity training exacerbates hypoxia in the renal medulla. In contrast, a report has shown that low/moderate intensity is not harmful [6]. Particularly, when autoregulation is impaired due to disease, reduced blood flow during exercise leads to renal hypoxia. Hypoxia is a vital factor in the development and progression of chronic kidney disease (CKD) [12].

The present study confirms that RBF was maintained at pre-VT exercise intensity, including aerobic exercise. Aerobic exercise can improve renal function tests and lipid profiles and slow the progression of stages 3 and 4 CKD [24]. In addition, more than half (6/10) of all studies conducted at moderate intensity tended to show a decrease in proteinuria [19]. Moderate exercise does not increase sympathetic activity due to the cardiopulmonary baroreflex, thus, suppressing the rise in sympathetic activity [19]. However, when exercise is above moderate intensity, increased central and peripheral nerve activity stimulates ventilation, cardiovascular function, and other functions [1].

Changes in autonomic nerves affect the HR and blood vessels. Sympathetic hyperactivity causes renal blood vessels to constrict [10]. In addition, angiotensin II (Ang II) has a significant effect on the contraction of blood vessels. The vasopressor action of Ang II involves an increase in peripheral vascular resistance due to the vasoconstriction of vascular smooth muscles, which is associated with the release of noradrenaline (NA) from sympathetic nerve endings. Furthermore, a high correlation was observed between the plasma NA value and lactate threshold [29]. The activation of these NA, norepinephrine, and epinephrine causes a decrease in the glomerular filtration rate and the RBF [5]. Our findings at pre-VT showed an increase in HR, but no changes in blood pressure, TAV, or CSA. Based on this, we concur that changes did not occur in the pre-VT stage. Consequently, exercise at pre-VT was capable of maintaining the RBF. Notably, exercising at 50%–60% of \(\dot{\text V}\)O2 max exceeding O2 max causes renal vasoconstriction with a detectable increase in renal nerve activity, circulating catecholamines, and plasma renin activity [9]. Renal arterial blood flow is affected by renal sympathetic nerve activity (RSNA) over its entire range, thereby inducing contraction and dilation of the renal vessels [25]. Therefore, we believe that the contraction of these systemic blood vessels and renal arteries contributed to the decrease in RBF at post-VT. Additionally, post-VT showed a decrease in RBF to 289 ± 111 mL/min from 461 ± 142 mL/min at baseline. The HR was 115 ± 12.6 beats/min, which was approximately twice of that at rest. The SBP significantly increased at post-VT, and the CSA also changed to 0.19 cm2 from 0.27 cm2 at baseline. Therefore, blood flow changes during the contraction of systemic blood vessels and renal arteries.

In this study, a decrease in TAV was observed at this load intensity, and a significant change in CSA was also confirmed. This result is similar to that of a previous study [15]. Furthermore, HR and SBP were significantly elevated post-VT, suggesting that vascular reactivity was enhanced after VT, corroborating the changes in RBF. In addition, since the HR and SBP significantly increased at RCP, we concur that they were affected by changes in the autonomic nerves, as seen during the post-VT stage.

The risk of kidney injury only increases when exercise intensity exceeds the lactate threshold [19]. This reduced renal arterial blood flow response after VT has important implications in renal disease patients with renal autoregulation disorders. However, patients with heart failure or renal failure with heart failure do not necessarily have a coincident decrease in blood flow due to abnormal cardiac pump function. At least in patients with the appearance of oscillatory ventilation on CPX, they do not necessarily have a concordant decrease in perfusion due to the difficulty in determining VT. The same is true for patients whose VT is difficult to determine, regardless of disease. Patients with renal failure have lower exercise tolerance than healthy subjects [7]. However, in patients with clearly identifiable VT, the exercise physiological response is similar [11] and may produce similar RBF changes. Therefore, a future research question is to confirm the blood flow changes associated with different diseases.

This study has some important limitations. First, ultrasonography did not accurately measure the RBF during CPX using the cycle ergometer. Therefore, it was used immediately after the participants completed the test. Second, the ultrasonography probe could not be used in a ventral approach. Third, participants with obesity could not provide stable measurements and had to be excluded from the study. This study observed that the exercise intensity that maintains RBF was that at pre-VT. However, it is unclear whether this applies to patients with comorbidities; in particular, the effect of moderate-intensity loading on renal artery blood flow in patients with cardiac disease is a topic for future research. Finally, if the blood distribution or volume of a patient differs from that of healthy subjects, the blood flow response to exercise may diverge from that of healthy subjects. Furthermore, there is insufficient evidence for blood distribution around the VT in patients with kidney disease, which cannot be substantiated by studies in healthy subjects, and this is a major limitation of this study.

Conclusions

This study focused on RBF measured using ultrasonography immediately after the participants used the cycle ergometer. The findings showed that exercises with intensity up to the vicinity of the VT could be performed without decreasing the RBF in healthy people.